1. Field of the Invention
The present invention generally relates to projecting a two-dimensional image of high quality, especially in color, using high power lasers and scan mirror drives, without exceeding governmental regulatory exposure limits on laser power, thereby enhancing viewer and bystander safety during image projection.
2. Description of the Related Art
It is generally known to project a two-dimensional image on a projection surface based on a pair of scan mirrors which oscillate by respective mirror drives over scan angles in mutually orthogonal directions to scan a laser beam over a raster pattern comprised of a plurality of scan lines. The image is created in the raster pattern by modulating a laser on and off at selected times, thereby illuminating selected pixels and not illuminating other pixels in each scan line. Color images can be obtained by modulating red, blue and green lasers and illuminating the selected pixels by superimposing one or more of the red, blue and green laser beams emitted by the respective lasers on a respective pixel to obtain a desired color.
The output power of each laser must be monitored for safety. Although the image brightness is enhanced when the output power of each laser is increased, government regulatory safety standards dictate the maximum power output of each laser for human safety. Each of the output powers of the red, blue and green laser beams must therefore be less than a maximum value. Some of these standards require that the output power of each laser does not exceed regulatory limits even when control circuitry that normally regulates the laser output power fails.
For example, an internal monitor photodiode inside the laser housing is normally operative for monitoring the laser output power. The monitor photodiode is part of a feedback circuit for maintaining the laser output power constant during operation. If the monitor photodiode were to fail, or to become electrically disconnected from the feedback circuit, then the feedback signal would be lost, and the feedback circuit would increase the laser output power, possibly to a level exceeding regulatory limits and compromise viewer and bystander safety.
The oscillation of each mirror must also be monitored for safety. For example, the scan angle of each mirror must be greater than a minimum value to insure that the drive for the respective mirror has not malfunctioned or stalled. This would cause the laser beams to dwell in an angular area smaller than the desired scan angle, or even remain stationary, and exceed the output power regulatory limits at that smaller angular area since the output power is more intense.